Titan's smile and collar: HST Observations of seasonal change 1994-2000

We observed Titan with the Hubble Space Telescope in November 2000 using the Wide‐Field Planetary Camera (WFPC2) and the Space Telescope Imaging Spectrograph (STIS). Considerable change is apparent between these and observations in 1994‐1997 : in particular the North‐South asymmetry at visible wavelengths has reversed, with the southern hemisphere brighter, as during the Voyager epoch. The asymmetry shows considerable variation with wavelength in the near‐infrared : in the 889nm methane band the asymmetry (which formerly resembled a ‘smile’) has reversed in the last 3 years, while at 953nm the southern limb is still bright. A south polar collar, dark at ultraviolet wavelengths, is apparent some five years after equinox.